Method and device for anaerobically fermenting organic material

ABSTRACT

Method for anaerobically fermenting biodegradable organic material, whereby this material is mixed with a quantity of already fermented material as an inoculum for the active anaerobic fermentation and this mixture is introduced at the top into a fermentation chamber ( 1 A) in which a fermenting mass is situated, which, while being fermented, moves from an inlet ( 6 A) situated at the top towards an outlet ( 8 ) situated at the bottom. Before the mixture is added to the fermenting mass, it is expanded beforehand until its density approximately coincides with the density of the already present fermenting mass at the beginning of this downward movement.

BACKGROUND

1. Field

This invention relates to a method for anaerobically fermentingbiologically degradable organic material, whereby this material is mixedwith a quantity of already fermented material as an inoculum for theactive anaerobic fermentation, and whereby this mixture is introduced atthe top into a fermentation chamber in which a fermenting mass issituated, which moves from an inlet situated at the top towards anoutlet situated at the bottom.

B. Related Art

By organic material, here in particular the organic fraction of domesticwaste is intended, and of similar industrial waste and other organicfractions, such as, for example, slurry from water purificationinstallations, slurry from the paper industry, or other kinds of organicslurry.

Methods for anaerobically fermenting organic waste can be divided intowet and dry manners of fermentation.

With wet fermentation methods, fresh or recycled water is added to thesolid organic waste or slurry in order to form a highly liquid mash orslurry which can be pumped easily and which can easily be mixed in afermentation tank. Such mash with 6 to 10% of dry matter, depending onthe viscosity of the starting material to be treated, is pumped into afermentation tank and is transformed into biogas, in mixed fermentationsystems, in one or two phases and with mesophilic or thermophilictemperatures.

Due to the high water content of the material in the fermentation tank,the density in this tank is relatively uniform, and the produced biogascan easily escape from the mixed mash.

With a dry fermentation, on the contrary, the quantity of water which isadded is limited, such that a relatively solid biologically degradablewaste is pumped or pushed into a “dry” fermentation tank by means ofspecial pumps. For organic fractions originating from domestic waste,the dry matter content of the material fed into the tank is between 15and 45%. For more viscous materials, this may be up to 10 to 15%.

In most cases, with dry fermentation the material is pumped at thebottom into the standing tank, whereby gas is injected in order toobtain a mixing and a horizontal movement of the material. The tank mayalso be installed horizontally and may be provided with a mixing devicewhich realizes the horizontal movement.

Such method of introducing the material at the bottom of a standing tankis described in WO 86/05200. Biogas is brought into the fermentationtank at different locations, through the underside, such that thisunderside is divided into sectors. The biogas provides for a mixing ofthe contents of the sectors and a movement of the fermenting mass fromone sector to the other and finally towards the outlet, also situated atthe bottom.

EP-A-0.476.217 describes a method, whereby a horizontal fermentationtank with therein a mixing device is used. After having been mixed witha portion of fermented material, the biodegradable material is pushed,through a feeding tube realized as a heat exchanger, into the tank andin this tank is mixed by a blender, as a result of which the fermentingmass flows horizontally through the tank, from one extremity to theother, where it is discharged at the bottom.

According to EP-A-0.205.721 and EP-A-0.577.209, the fermentation isperformed in a vertical fermentation tank, without a mixing devicetherein. The biodegradable material is mixed with fermented material asan inoculum and is pumped into the tank at the top.

In the tank, the fermenting mass sinks, as fermenting material isdischarged at the bottom. This latter is performed by means of a slidinggrid moving to and fro above the flat bottom of the tank, which gridpushes this fermented material into a screw conveyor device situatedtherebelow. This device pushes the fermented material into another screwfunctioning as a lock by forming a stopper of fermented material at itsconical outlet.

In the method according to EP-A-0.577.209, moreover, due to the designof the device, a fraction division into a liquid and a solid fraction isobtained, such that the content of solid matter in the fermentation tankcan be maintained and that the dry matter content of the fed material,which consists of a mixture of inoculum and fresh material, is situatedbetween 15 and 40%.

In these fermentation tanks without mixer, it is typical that thecontent thereof rises and expands as a result of the produced biogas.This biogas can not bubble immediately upward as in a wet tank and cannot easily escape, due to the high viscosity of the material in which itis created and through which it must weave its way upward.

In practice, the material will expand as a consequence thereof, and itsdensity will be reduced by about 10 to 40%. The average density dependson various parameters, such as the degree of biogas production, thefeeding frequency, the kind of structure of the added material, and theheight of the feeding tank. Depending on the feeding, the density mayvary from 0.7 to 1.2 kg/l.

It was noted that this feeding matter, being a mixture of freshbiodegradable waste and fermented material or residue, has a densitywhich mostly is higher than 1.0 kg/l, on account of the fact that theprevailing biogas during mixing of the fermented material and the freshmaterial can escape from the fermented material.

When this feeding matter with fresh material and inoculum, which, due tothe degassification, has obtained a higher density, is brought at thetop into the fermentation tank in which the material has a lowerdensity, then, depending on the difference in density, the dimensionsand the diameter of the fermentation tank and the extraction systemthereof, too fast a sinking of this feeding matter through the mass inthe tank can be noted, such that this feeding matter arrives faster atthe bottom than the remainder of the fermenting mass and, thus, isdischarged from the tank without having undergone an optimumfermentation.

BRIEF SUMMARY OF THE INVENTION

The invention aims at a method which does not have these disadvantagesand whereby the mixture of fresh material and fermented material isbrought at the top into a vertical fermentation chamber in whichmaterial moves downward from the top, however, whereby it is avoidedthat said added mixture sinks faster than the remainder of thefermenting mass.

According to the invention, the mixture, before being added to thefermenting mass and further moving downward during fermentation, isexpanded beforehand until its density approximately coincides with thedensity of the already fermenting mass at the beginning of this downwardmovement.

Preferably, the mixture is beforehand expanded or reduced in density,while being moved upward.

Preferably, a mixture is made of one quantity of fresh material,together with one to ten quantities of fermented material.

Preferably, a mixture is treated, having a dry matter content of 10 to45%.

The downward movement of the fermenting mass in the fermentation chambercan take place in a fermentation tank which is free of any mixer,whereby the mixture is supplied at the upper side of the fermentationtank, after it has been reduced in density in an expansion vessel.

The mixture can be expanded beforehand by means of a precedingbiological pre-fermentation, by injecting a gas into the mixture, bybeating this mixture or also by adding chemicals which lead to therelease of gasses in the mixture, or by a combination of two or more ofthese procedures.

If the expansion or density reduction is performed by means ofbiological pre-fermentation, this can be performed in an expansionvessel in which the mixture remains at least 5 minutes, and preferablybetween 15 minutes and 3 hours.

The sojourn time in the expansion vessel also may be more than threehours, for example, 3 to 72 hours.

The invention also relates to a device which is particularly suitablefor applying the method according to any of the preceding forms ofembodiment.

Consequently, the invention relates to a device for anaerobicallyfermenting organic waste, which device comprises a fermentation tankwith a fermentation chamber, an outlet for fermented material at thebottom of this tank, and a biogas outlet at the top and a feeding devicecomprising a mixer and which, at the top, gives out into thefermentation tank, and which is characterized in that the feeding devicecomprises at least an expansion vessel situated between the mixer andthe fermentation tank.

The feeding device also may comprise a pump to which the mixer givesout, whereby the expansion vessel is situated between the pump and thefermentation tank.

This expansion vessel may be situated at least partially in thefermentation tank as well as at the exterior side thereof.

The expansion vessel may be reduced to a supply conduit.

DESCRIPTION OF THE DRAWINGS

The device may comprise a device for gas injection or an inlet forchemicals which lead to the release of gasses, which latter connects tothe expansion vessel, the mixer or a pump connected thereto, or abeating device can be provided in the feeding device.

With the intention of better showing the characteristics of theinvention, hereafter, as an example without any limitative character,several preferred forms of embodiment of a method and device foranaerobically fermenting organic material according to the invention aredescribed, with reference to the accompanying drawings, wherein:

-   -   FIG. 1 schematically represents a vertical cross-section of a        device for anaerobic fermentation according to the invention;    -   FIG. 2 represents a cross-section according to line II—II in        FIG. 1;    -   FIG. 3 schematically represents a cross-section analogous to        that of FIG. 1, however, relating to another form of embodiment;    -   FIG. 4 represents a cross-section according to line IV—IV in        FIG. 3;    -   FIGS. 5 to 9 represent schematic cross-sections analogous to        those of FIGS. 1 and 3, however, relating to still other forms        of embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The device for anaerobically fermenting organic material, represented inFIGS. 1 and 2, substantially consists of a closed fermentation tank 1,which comprises a fermentation chamber 1A, and a feeding device 2comprising a transport device 3, a mixer 4, a pump 5 and an expansionvessel 6, which, at the top, gives out into the fermentation tank 1. Theoutlet of the expansion vessel 6 at the top forms the inlet 6A of thefermentation tank 1.

The inlet for material in the fermentation tank 1, thus, is situated atthe top, and the outlet 8, which can be closed off by a valve 7, issituated below the funnel-shaped bottom of this fermentation tank 1.This fermentation tank 1 also may have a flat bottom with an outlet 8 inthe bottom, possibly by means of screws of other extraction systems, oran outlet 8 at the underside of its wall.

In the outlet 8, as represented, transport screws 9 can be installed.The mixer 4 is connected to this outlet 8 and is situated below anextremity of the transport device 3, which, in the represented example,is formed by a transport conveyor. By means of this transport conveyoror another transport device, such as a screw conveyor, fresh organicmaterial can be added to the mixer 4.

Downstream of the valve 7, a branch 10, which can be closed off by avalve 11, connects to the outlet 8, between two screws 9. In the branch10, downstream of the valve 11, a transport screw 12 can also bearranged. The branch 10 connects to a dewatering device 13.

To the mixer 4, a water conduit 15, which can be closed off by means ofa valve 14, is connected for possibly supplying water to the mixer 4, aswell as a steam conduit 17, which can be closed off by a valve 16, forinjecting steam in order to heat the material.

The outlet of the mixer 4 connects to the inlet of the pump 5, whereasthe outlet of the pump 5, on one hand, connects to a conduit 19, whichcan be closed off by a valve 18, which conduit 19 serves fortransporting off material, and, on the other hand, connects to theunderside of the expansion vessel 6, by means of a conduit 20 whichpossibly can be closed off by a valve 20A.

The expansion vessel 6 penetrates into the fermentation tank 1 throughthe funnel-shaped bottom thereof and, with its opening, gives out intothe fermentation tank 1 above the upper side of the inlet 6A, below orabove the level 21 of the fermenting mass in this tank 1.

The expansion vessel 6, for example, is round, as well as thefermentation tank 1, but has a much smaller diameter.

At the top, an outlet 22 for biogas connects to the fermentation tank 1.

Fresh material to be treated is introduced into the mixer 4 by means ofthe transport device 3, together with a part of the fermented materialwhich is coming through the opened valve 7 out of the fermentation tank1 and further is transported by the screws 9 or another transport deviceto the mixer 4.

In the mixer 4, the fresh material and the fermented material as aninoculum are mixed in a ratio of one quantity of fresh material for oneto ten quantities of fermented material.

The dry matter content of the mixture is maintained between 10 and 45%and preferably between 15 and 45% and, if necessary or desired, aquantity of water, for example, waste water, is brought into the mixer 4through water conduit 15. This water can be supplied hot in order tobring the final mixture to a temperature of 30 to 42° C. (mesophilic) or45 to 60° C. (thermophilic). If no water must be added, steam can beinjected, by means of the steam conduit 17, into the mixer 4 in order tobring the temperature onto the desired level.

Subsequently, the mixture produced in the mixer 4 is pumped to theexpansion vessel 6 through conduit 20. Valve 18 is closed and valve 20Ais open. In this expansion vessel 6, the mixture moves upward, asindicated by arrow P1, whereas it undergoes a biologicalpre-fermentation with the production of biogas.

As a consequence thereof, the mixture will rise in the expansion vessel6 and decrease in density. The sojourn time of the mixture in thisexpansion vessel 6 is at least 10 minutes and preferably is situatedbetween 30 minutes and 3 hours.

This sojourn time is chosen such that, when, due to the pumping in ofnew mixture, the mixture is pushed out of the expansion vessel 6 andarrives at the top of the fermentation tank 1, its density isapproximately equal to the density of the fermenting mass which alreadyhas been present at the top of this fermentation tank 1.

As a result of the formation of biogas, the density of thispre-fermented supplied mixture, due to rising, is lower than the densityof the fresh supplied mixture which is pumped by the pump 5 from themixer 4 into the expansion vessel 6.

In the fermentation tank 1, the further fermentation takes place, andthe fermenting mass sinks towards the outlet 8, where it leaves thefermentation tank 1 as fermented material. The formed biogas isdischarged through outlet 22.

By opening the valve 11, fermented material from outlet 8 gets intobranch 10. This material is transported by the screw 12 to thedewatering device 13, where it is separated into press water 13A and apress cake 13B, which are transported off.

It is clear that the device may comprise more than one expansion vessel6, whereby the expansion vessels 6, for example, three expansion vessels6, are connected to the pump 5, and whereby preferably, they are equalto each other and installed in the same manner. In FIGS. 1 and 2, twoadditional expansion vessels 6 are represented in dashed line.

The form of embodiment represented in FIGS. 3 and 4 differs from thataccording to FIGS. 1 and 2 in that the expansion vessel 6, expansionsvessels 6, respectively, if there are several, is or are provided at theexterior side on the fermentation tank 1.

In FIG. 3, moreover in dashed line a conduit 23 is represented, whichforms a bypass of the expansion vessel 6, which bypass, on one hand,connects to the conduit 19 and, on the other hand, branches off inbranches 23A which give out onto the upper side of the fermentation tank1 and can be closed off by a valve 24. The outlets of the branches inthe tank 1 form the inlet 6A of this tank 1.

The functioning is as described above, however, if desired, throughbypass 23, the mixture of inoculum and fresh material can be broughtwithout expansion into the fermentation tank 1.

The expansion of the mixture of inoculum and fresh material does notnecessarily have to take place by means of a pre-fermentation in anexpansion vessel 6. This expansion may also be obtained by injectinggas, for example, part of the collected biogas, into the mixture,whereby, when being introduced into the fermentation chamber 1A, themixture immediately expands, and the density thereof becomes equal tothe density of the fermenting mass at the top of the fermentationchamber 1A.

This gas can be injected into the expansion vessel 6, however, thisexpansion vessel 6 may also be reduced to a normal conduit, for example,only the bypass 23 with the valves 24 of the form of embodimentaccording to FIGS. 3 and 4, whereby the gas is brought under pressureinto the mixture.

In FIG. 5, such device is represented, whereby an injection conduit 25discharges into conduit 23, which can be closed off by a valve 26.

The same effect can be obtained by, instead of injecting gas, injectinga chemical agent which causes gas production in the mixture. In FIG. 5,in dashed line an inlet 27 of such chemical agent is represented, whichconnects to conduit 23 and can be closed off by a valve 28. Thischemical agent which causes gas production, possibly can be added in themixer 4 or in the pump 5.

Still another possibility for reducing the density of the mixture offresh material and inoculum supplied to the fermentation chamber 1A andfor expanding this mixture, consists in beating this mixture, forexample, in the expansion vessel 6.

In this case, too, the expansion vessel 6 can be a reduced conduit 23,whereby the beating can take place by beating devices 29 in chambers 30in the branches 23A of this conduit 23, as represented in FIG. 6.

The expansion vessel 6 does not necessarily have to extend over theentire height of the fermentation tank 1. It may, for example, be avessel which connects to the fermentation tank 1 at the exterior, athalf the height.

The device does not necessarily have to comprise a pump 5. Instead of apump, it may comprise another transport mechanism, such as a screw orthe like.

The expansion vessel 6 may also be formed by one or more compartments32, defined by partitions 31, of a vertical tank 33, whereby then thespace outside of the compartment 32 or the compartments 32 forms, form,respectively, the fermentation tank 1 with the fermentation chamber 1A.

It is essential that the compartment 32 or the compartments 32discharges or discharge respectively, at the top of the fermentationchamber 1A, and allow for a certain upward flow of the mixture to befermented.

In FIG. 7, a device with such tank 33 is represented. A partition 31,situated above the conical bottom of the tank 33, together with a partof the wall of the tank 33 forms a compartment 32 which is closed at thebottom and open at the top. The mixture of fresh material and inoculumis introduced into this compartment at the bottom, where it is expandedby means of biological pre-fermentation.

In FIG. 8, a similar device is represented, however, in this case thepartition 31 is directed upwardly inclined, starting from the inside ofthe tank. As represented, this ring can be provided over the entireinner circumference of the tank 33, such that the compartment thus formsa circle-shaped gutter, or can be provided locally, such that severaltrough-shaped compartments 32 are formed at the interior side of thetank 33, which latter compartments form an expansion vessel 6.

The expansion vessel 6 may not only be situated at an interior side ofthe fermentation tank 1, as represented in FIGS. 1 and 2, however, itmay also be situated centrally, around the fermentation tank 1, asrepresented in FIG. 9.

This fermentation tank 1 then is formed by a tubular element which isopen at the top and which protrudes with one outlet 8 through thefunnel-shaped bottom of expansion vessel 6.

The invention is in no way limited to the forms of embodiment describedin the aforegoing and represented in the figures; on the contrary, suchmethod and device for anaerobically fermenting biodegradable materialmay be realized in different variants, without leaving the scope of theinvention.

1. Method for anaerobically fermenting biodegradable organic material,wherein the organic material is mixed with a quantity of alreadyfermented material as an inoculum for the active anaerobic fermentationto produce a mixture and the mixture is introduced at the top into afermentation chamber in which a fermenting mass is located and whichmixture, while being fermented, moves from an inlet located at a top ofthe chamber towards an outlet located at a bottom of the chamber,comprising the steps: expanding the mixture before the mixture is beingadded to the fermenting mass; and moving the expanded mixture downwardduring fermentation until the mixture density approximately coincideswith the density of fermenting mass in the chamber at the beginning ofthe downward movement.
 2. Method according to claim 1, wherein themixture beforehand is expanded or is reduced in density, while beingmoved upward.
 3. Method according to claim 1 wherein the mixture is madeof one quantity of fresh material, together with one to ten quantitiesof fermented material.
 4. Method according to claim 3, wherein themixture comprises biologically fresh material and fermented material,and the mixture has a dry matter content of 10 to 45%.
 5. Methodaccording to claim 1, wherein the downward movement of the fermentingmass in the fermentation chamber is carried out in a fermentation tankwhich is free of any mixer, and the mixture is introduced at the upperside of the fermentation tank after it has been expanded in an expansionvessel.
 6. Method according to claim 1, wherein the mixture is expandedby a method selected from the group consisting of: a precedingbiological pre-fermentation; an injection of a gas into the mixture; abeating of the mixture; and an addition of chemicals to the mixturewhich causes the formation of gas in the mixture, and combinationsthereof.
 7. Method according to claim 6, wherein the expansion isperformed by a biological pre-fermentation carried out in an expansionvessel in which the mixture remains at least 5 minutes.
 8. Methodaccording to claim 6, wherein the sojourn time of the mixture in theexpansion vessel during expansion is 3 to 72 hours.
 9. Device foranaerobically fermenting organic waste, comprising a verticalfermentation tank, an outlet for fermented material at a bottom of saidtank, and an outlet for biogas at a top of the tank, and a supply devicecomprising a mixer which discharges into the fermentation tank at thetop thereof, wherein the supply device comprises at least one verticalexpansion vessel which is located between the mixer and the fermentationtank.
 10. Device according to claim 9, wherein the supply devicecomprises a pump in communication with the mixer and wherein theexpansion vessel is located between the pump and the fermentation tank.11. Device according to claim 9, wherein the expansion vessel is locatedat least partially within the fermentation tank.
 12. Device according toclaim 9, wherein the expansion vessel is installed on an exterior sideof the fermentation tank.
 13. Device according to claim 9, wherein theexpansion vessel comprises a compartment defined by at least onepartition and the interior wall of a tank, and a chamber of the tanklocated next to the compartment forms the fermentation tank.
 14. Deviceaccording to claim 9, wherein the expansion vessel completely surroundsthe fermentation tank.
 15. Device according to claim 9, wherein theexpansion vessel comprises a conduit.
 16. Device according to claim 9,including an injection device for gas, or an inlet for at least achemical agent which causes gas production, said injection device orinlet connected to the expansion vessel, to the mixer or to a pumpconnected to the mixer.
 17. Device according to claim 9, including abeating device in the expansion vessel.
 18. Device according to claim15, wherein the conduit includes branches, discharging onto the upperside of the fermentation tank and wherein the branches include, chamberseach having a beating device therein.
 19. Device according to claim 9,wherein the fermentation tank has a flat bottom.